Polysilazanes and related compositions, processes and uses

ABSTRACT

Silazanes and related compounds are prepared by providing a precursor containing one or more cyclic silazane units, causing polymerization to occur in the presence of a transition metal catalyst to form a polysilazane product. Further products may result from additional reaction. The novel compounds may be pyrolyzed to yield ceramic materials such as silicon nitride, silicon carbide and mixtures thereof. In a preferred embodiment, substantially pure silicon nitride and articles prepared therefrom are provided. Fibers, coatings, binders and the like may be prepared from the novel materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.07/176,824, filed Apr. 4, 1988, now U.S. Pat. No. 4,952,715, which was acontinuation-in-part of U.S. application Ser. No. 07/012,874, Filed Dec.1, 1986, which was a continuation-in-part of U.S. application Ser. No.06/908,685, filed Mar. 4, 1986, now U.S. Pat. No. 4,788,309, which was acontinuation-in-part of U.S. application Ser. No. 06/727,415, filed Apr.26, 1985, now U.S. Pat. No. 4,612,383.

TECHNICAL FIELD

The invention relates to the synthesis of silazane compounds, i.e.,compounds containing the structure Si-N in the molecule, and primarilyconcerns silazanes and polysilazanes which have one or more cyclomericsilazane units in their structure. The invention also relates (1) to thepyrolysis of these compounds after fabrication to give ceramic coatings,fibers and articles as well as (2) to the use of these compounds asbinders.

BACKGROUND

Polysilazanes and their derivatives are useful among other things, forthe preparation of silicon nitride (Si₃ N₄), silicon carbide (SiC), Si₃N₄ /SiC alloys, Si₃ N₄ /carbon alloys, Si₃ N₄ /boron nitride alloys, andmixtures thereof. These ceramic materials can be used as structuralmaterials, protective coatings, and electronic materials because oftheir hardness, strength, structural stability under extremeenvironmental conditions and their wide variety of electronicproperties. In particular, these materials can be formed into ceramicfibers of value for reinforcement of composite materials. See, forexample, (a) Department of Defense Proceedings, Fourth Metal MatrixComposites Technical Conference, May 19-21, 1981, prepared for DOD MetalMatrix Composites Information Analysis Center; and (b) J. J. Brennan,"Program to Study SiC Fiber-Reinforced Matrix Composites", Annual Reportto Dept. of Navy (Nov. 1980), Contract No. N00014-78-C--0503.

Historically, polysilazanes were first synthesized by Stock et al almost60 years ago (see, e.g., Stock, A. and K. Somieski, Ber. Dtsch. Chem.Ges. 54:740 (1921)) via a simple ammonolysis technique (Scheme I).However, this ##STR1## approach usually produces mixtures of cyclomerswhere x is 3 to 5 that are obtained as the major products and smallamounts of linear oligomers where y is less than or equal to about 10.Because of their low molecular weight, however, these linearoligosilazanes are too volatile to be used as preceramic materials.

In order to obtain higher molecular weight, nonvolatile materials, itwas necessary to promote crosslinking reactions. In this manner,moderate molecular weight polysilazanes have been synthesized using avariety of techniques. See, e.g., Kruger, C. R. and E. G. Rochow, J.Polymer Sci. 2A:3179-3189 (1964). Rochow et al. discovered that ammoniumchloride catalyzes crosslinking in simple oligodimethylsilazanes to formpolysilazanes (Scheme II) which ##STR2## were proposed to contain cyclicmonomer units crosslinked through nitrogen as suggested by the structure##STR3##

The Penn et al. work follows up on U.S. Pat. No. 3,853,567 to Verbeekand U.S. Pat. No. 3,892,583 to Winter et al., wherein a high temperatureelimination/condensation reaction was shown to lead to soluble, highlycrosslinked polymers as shown in Scheme III. Pyrolysis at hightemperatures provides ceramic ##STR4## yields of 60% with a mixture ofSi₃ N₄ and SiC ceramic materials.

A related crosslinking approach described, inter alia, in U.S. Pat. Nos.4,312,970, 4,340,619, 4,535,007 and 4,543,344 begins with thepreparation of tractable polysilazanes having Me₃ Si groups in thepolymer backbone (Scheme IV) with the highest molecular weights reportedin the available literature, i.e., about Mw˜15,000 D and Mz˜39,000 D:##STR5## Ceramic yields obtained from pyrolysis of this polymer are onthe order of 45-55% with compositions of 96% Si₃ N₄, 2% carbon and 2%oxygen after curing.

U.S. Pat. No. 4,482,669 to Seyferth et al. discloses that it is possibleto crosslink low molecular weight cyclic oligomers containing Si-H bondsadjacent to N-H bonds via the following reaction: ##STR6## The NH bondis catalytically activated by the strong base in this reaction. Thistype of crosslinking generates two-dimensional polymers, the solubilityof which is limited by their sheet-like character. Ceramic yields ofthese materials are often quite high, up to about 86%, and typicallyprovides Si₃ N₄, SiC and carbon in a mole ratio of 0.88:1.27:0.75. Ifthe pyrolysis is carried out in an NH₃ atmosphere, then the only productis Si₃ N₄ with the other products remaining as slight impurities.

Zoeckler and Laine in J. Org. Chem. (1983) 48:2539-2541 describe thecatalytic activation of the Si-N bond and in particular the ring openingof octamethylcyclotetrasilazane and polymerization of the ring-openedintermediate. Chain termination is effected by introducing [CH₃)₃ Si]₂NH as a coreactant giving rise to polymers (CH₃)₃ Si(CH₃)₂ ]_(n)-NHSi(CH₃)₃ where n may be 1 to 12 or more depending upon the ratio ofthe chain terminator to the cyclic silazane. The catalyst used was Ru₃(CO)₁₂. Other publications are as follows W. Fink, Helv. Chem Acta.,49:P1408 (1966); Belgian Patent 665774 (1965); Netherlands Patent6,507,996 (1965); D. Y. Zhinkis et al., Rus. Chem. Rev., 49:2814 (1980);K. A. Andrianov et al., Dok Akad. Nauk SSSR, 227:352 (1976); Dok Akad.Nauk. SSSR 223:347 (1975); L. H. Sommer et al., JACS 91:7061 (1969); L.H. Sommer, J. Org. Chem. 32:2470 (1967); L. H. Sommer et al., JACS89:5797 (1967).

In general, control of the polysilazane molecular weight, structuralcomposition and viscoelastic properties plays a considerable role indetermining the tractability (solubility, meltability or. malleability)of the polymer, the processability during. fabrication of fibers, shapedarticles, etc., the ceramic yield, and the selectivity for specificceramic products. In particular, the tractability plays a major role inhow useful the polymer is as a binder, or for forming shapes, coatings,spinning fibers and the like. The more crosslinked a polymer is, theless control one has of its viscoelastic properties. Thus, highlycrosslinked, low molecular weight polymers that are rigid materials orgels are not particularly useful for spinning fibers or as bindersbecause they lack the required flexibility, viscosity and tenacity. Bycontrast, high molecular weight, flexible polymers as provided byapplicants are extremely important. Such polymers represent asignificant advance in the art, as they provide the flexibility andtenacity necessary in the fiber-spinning process and enhance the overalltensile strength of the spun fibers. In addition, the viscosities andthe softening and melting points of the novel polymers may play a keyrole in binder applications, and in injection-molding processes inparticular.

The parent application hereto, U.S. application Ser. No. 012,874, filedDec. 1, 1986, describes the preparation of such high molecular weight,substantially linear polysilazanes. The disclosure of that applicationis hereby incorporated by reference in its entirety, as that applicationdescribes in some detail various compounds, methods and uses relevant tothe present invention but not explicitly addressed herein.

The present application is directed to a subset of the compositions andmethods described and claimed in Ser. No. 012,874. Specifically, thepresent application is directed to silazanes and polysilazanes thatinclude at least one cyclomeric silazane unit in the molecularstructure.

As discussed above, several routes to polymers containing the monomericunits -[MeSiHNH]- have been developed. Ammonolysis of MeSiHCl₂ generatesthe low molecular weight cyclomer 1-cyclomethylsilazane (CMS) ##STR7##which, as a non-viscous liquid that gives ceramic yields on the order of20 wt. %, is impractical as a ceramic precursor. Arai et al. (see U.S.Pat. No. 4,659,850) have demonstrated how a modification of theammonolysis process results in a higher molecular weight species TheArai et al. group reacted dichloromethylsilane with ammonia in thepresence of pyridine at 80° C. The Lewis base complexes with thechlorosilane and causes the formation of a linear-cyclomer copolymerhaving trisilylated nitrogen bridges (M_(n) =1100-1800 D, ceramic yieldabout 44 wt %). Similarly, Matsumoto et al. (Japan Patent Publication[Kokai] No. 61-72607) disclose reaction of a dihalosilane with ammoniawhich is stated to give relatively high molecular weight, highly viscouspolysilazanes.

The Seyferth et al. polymers (U.S. Pat. No. 4,482,669, cited supra)formed by dehydrocyclodimerizing CMS oligomers, are, as noted above,very rigid structures. This rigidity, although responsible in part forthe high ceramic yields obtained upon pyrolysis, prevents the polymerfrom having softening or melting points. Such polymers with an M_(n)over about 2500 D are brittle, intractable gels. No liquid polymers,even of low molecular weight, have been formed by this KH catalysismethod.

The presently claimed compounds are believed to include the firstreported polymers of [MeSiHNH]_(n) that are either liquid or have asoftening or melting point, indicating higher structural flexibility andperhaps higher linearity as well. Like the parent application hereto,the present disclosure demonstrates how transition metal catalysis maybe used for modifying the characteristics of inorganic polymers and,specifically, how control over polymer properties, pyrolysis results andthe final ceramic compositions can follow directly from the selection ofthe precursor, the chosen chemical method and the reaction conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are SEM photographs of the polysilazane fiber prepared asdescribed in Example 22.

DISCLOSURE OF THE INVENTION

It is thus a primary object of the present invention to provide improvedmethods of preparing silazanes and, in particular, polysilazanes havingone or more cyclomeric silazane units in their molecular structure.

It is another object of the invention to provide novel compounds whichinclude such polysilazanes.

It is still another object of the invention to provide a method ofmaking a ceramic composition including silicon nitride and/or siliconcarbide, by pyrolyzing the polysilazane compounds provided herein.

It is a further object of the invention to provide a method of coatingsubstrates with ceramic materials as described herein.

Other objects of the invention include methods of making fibers, fine ormonodispersed powders, coatings, and the like, using the preceramicpolymers and the ceramic materials disclosed herein.

Still other objects of the invention include methods of using thepolymers of the invention as binders, as adhesives, in infiltrationapplications, and in matrix and composite materials.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art on examination of thefollowing, or may be learned by practice of the invention.

In one aspect of the invention, a polysilazane is prepared having nrecurring units given by structures (I) or (II) or both ##STR8## where mis 0 or an integer, n is an integer greater than 1, the R substituentsmay be the same or different and are selected from the group consistingof hydrogen, lower alkyl, lower alkenyl, silyl, aryl including phenyl orbenzyl, and amino, unsubstituted or substituted with 1 or 2 lower alkylor silyl groups, X is --NR'--or --NR'--Y--NR'--where Y is an optionallinking group which, if present, is lower alkyl or silyl (includingsilazanyl), and R' and R" are independently selected from the groupconsisting of hydrogen, lower alkyl, lower alkenyl, silyl includingsilazanyl, and aryl including phenyl or benzyl. The polysilazane isprepared by pyrolyzing a silazane precursor in a selected environment inthe presence of a transition metal catalyst, the precursor having thestructure ##STR9##

In other aspects of the invention, polysilazanes given by structures (I)or (II) are pyrolyzed to give ceramic products such as shaped articles,fibers, powders, and the like. The novel polymers in general displayimproved processability and thermolytic properties.

MODES FOR CARRYING OUT THE INVENTION A. Definitions

"Silazanes" as used herein are compounds which contain one or moresilicon-nitrogen bonds. The term "polysilazane" is intended to includeoligomeric and polymeric silazanes, i.e., compounds which include two ormore monomeric or cyclomeric silazane units.

The number average molecular weight M_(n) of a polymer distribution isgiven by ##EQU1## wherein Wi is the weight of each individual polymericor oligomeric species and Ni is the number of individual species in thedistribution. Where not otherwise specified, molecular weights for aparticular polymer distribution obtained directly will be given ascalculated prior to any separation or isolation step.

The "ceramic yield" of a compound upon pyrolysis indicates the ratio ofthe weight of the ceramic product after pyrolysis to the weight of thecompound before pyrolysis.

A "lower alkyl" or "lower alkenyl" group is an alkyl or alkenyl group,respectively, having 1-6 carbon atoms therein.

"Silyl" as used herein is an --SiR₂ --or --SiR₃ moiety where R ishydrogen, lower alkyl, lower alkenyl or amino, unsubstituted orsubstituted with 1 or 2 lower alkyl or lower alkenyl groups. "Silyl"moieties thus include "silazanyl" groups.

The parent application hereto defines the two types of reactions bywhich silazanes and polysilazanes are formed according to the method ofthe invention. In the type (a) reaction, a precursor is initiallyprovided which contains at least one Si--N bond. Cleavage of an Si--Nbond in the precursor is catalytically effected in the presence ofhydrogen or a hydrogen donor, and the cleavage product is then caused toreact with a second cleavage product or with a compound containing anSi--H bond, an N--H bond, or both, to produce an initial silazaneproduct having at least one newly formed Si--N bond.

In what applicants have referred to as the "type (b)" reaction, one ormore reactants are provided which in combination contain an Si-H bondand an N--H bond, and reaction is caused to occur between the two bondsin the presence of a transition metal catalyst, whereby an initialsilazane product is provided having at least two Si--N bonds, at leastone of which is newly formed. The polymerization of cyclomer-containingprecursors as described in the present application, as will bediscussed, falls within that class of reactions designated as "type(b)".

B. The Precursor Material

As described in U.S. Ser. No. 012,874, the precursor material may bemonomeric, oligomeric or polymeric. The precursor is a silazane havingone or more units of the formula ##STR10## where m is 0 or an integer,and the R substituents, which may be the same or different, are selectedfrom the group consisting of hydrogen, lower alkyl, lower alkenyl,silyl, aryl including phenyl or benzyl, and amino, unsubstituted orsubstituted with 1 or 2 lower alkyl groups. The precursor, in order toprovide the desired "bridged" product, must have either: (1) an Si--Hand an N--H bond (i.e., at least one R and at least one R" arehydrogen); or (2) 2 Si--H bonds (i.e., at least two R substituents arehydrogen). The precursor may be either a cyclic silazane as defined bythe above structure alone, or it may be a chain or network of suchcyclomers. In the latter case, the molecular weight of the precursor ispreferably less than about 2000 D. Such a chain or network may includeamine or silyl bridges between the cyclomers.

These precursors may be prepared by the method described in the parentapplication hereto, i.e., by reaction of a dihalogenated silane withammonia or an amine. In a particularly preferred embodiment, the1-cyclomethylsilazanes (CMS) given by the above structure are preparedby reaction of Me(H)SiCl₂ with ammonia, at low temperature (preferably0° C. or lower) in a suitable solvent. The "R" substituents areintroduced by appropriate selection of the dihalogenated silane, i.e.,by using R₂ SiCl₂ at the outset.

CMS is particularly suitable for dehydrocoupling catalysis by transitionmetals because of the multiple Si--H and N--H sites which enablebridging between the cyclomers. As will be addressed in some detail, thecharacteristics of the polymeric products obtained by transitionmetal-catalyzed dehydrocoupling are very different from those formed bydehydrocyclodimerization as described by Seyferth et al., supra. In analternative embodiment of the invention, the doubly bridged polymersdisclosed in U.S. Pat. No. 4,482,669 to Seyferth et al. are used asprecursors in the method herein to give polysilazane structures havingsingle as well as double bridges, i.e., given by the structure ##STR11##Such polymers are more flexible but nevertheless give high ceramicyields on the order of those obtained with the doubly bridgedprecursors.

The precursors described may also be modified prior to polymerization byinclusion of latent reactive groups such as hydrogen, amine, alkoxy,sulfide, alkenyl, alkynyl, etc., or crosslinked with suitablecrosslinking reagents.

C. Formation of Polysilazanes

Depending on the reactants and reaction conditions chosen, theabove-described precursors are polymerized catalytically to givepolysilazanes having n recurring units of either (I) or (II) or both##STR12## where m is 0 or an integer, n is an integer greater than 1,the R substituents may be the same or different and are selected fromthe group consisting of hydrogen, lower alkyl, lower alkenyl, silyl,aryl including phenyl or benzyl, and amino, unsubstituted or substitutedwith 1 or 2 lower alkyl or silyl groups, X is --NR'--or --NR'--Y--NR'where Y is an optional linking group which, if present is lower alkyl orsilyl (including silazanyl), and R' and R" are independently selectedfrom the group consisting of hydrogen, lower alkyl, lower alkenyl, silylincluding silazanyl, and aryl including phenyl or benzyl. Typically, "n"gives a molecular weight on the order of 500 to 10,000 D.

The polysilazanes so provided, depending on the reaction conditionschosen--i.e., on the presence and amount of ammonia or amine used,solvent and temperature--may be substantially linear, somewhat bridged,or highly crosslinked, and may contain repeating units of only structure(I), only structure (II), or an admixture of both. For example, a highernitrogen content may be effected by increasing the exposure of theprecursor to ammonia, which, as illustrated by Scheme VI, will introducemore nitrogen "bridges" into the polymer between individual cyclomerunits. A higher nitrogen content may also be achieved by increasing theexposure to a substituted amine, in which case, as illustrated by SchemeVII, nitrogen-containing substituents are introduced into the cyclomers(pendant to individual silicon atoms in the cyclomer rings) in additionto nitrogen bridging.

The optional amine co-reactant present in the polymerizationenvironment, may be generally represented by the formula γNH₂, where γis hydrogen, lower alkyl, silyl optionally substituted with one or morealkyl groups, or --(CH₂)_(x) NH₂ where x is an integer in the range of 0and 20 inclusive.

In all cases, the reaction is carried out catalytically.

Catalysts suitable for carrying out polymerization of these precursorsare any type of transition metal catalysts such as those indicated inTable 1, below, which are homogeneous catalysts that either dissolve inthe reactants or in a solvent used to dissolve the reactants.Heterogeneous catalysts such as those of Table 2 may also be used ormixtures of homogeneous catalysts and/or heterogeneous catalysts. (Itshould be pointed out here that the "homogeneous" and "heterogeneous"classifications are made herein on the basis of solubility in organicsolvents. However, it is not uncommon that during the reactions,homogeneous catalysts may be converted into a heterogeneous form andvice versa.) These catalysts may include any number of ligands,including carbonyl, amino, halo, silyl, hydrido, phosphine, arsine andorganic ligands. Tables 1 and 2 illustrate a number of catalysts whichmay be used.

The catalyst(s) may be supported on a polymer, inorganic salt, carbon,or ceramic material or the like. The heterogeneous catalyst may beprovided in a designed shape, such as particles, porous plates, etc.

The catalyst can be activated if desired such as by heating alone or byconcurrent treatment of the reaction medium with particulate ornonparticulate radiation. The catalyst may also be activated bypromoters such as acids, bases, oxidants or hydrogen, or may bestabilized by reagents such as amines, phosphines, arsines and carbonyl.The concentration of catalyst will usually be less than or equal toabout 5 mole % based on the total number of moles of reactants, usuallybetween about 0.1 and 5 mole %. In some instances, however, catalystconcentration will be much lower, on the order of 20 to 200 ppm.

                  TABLE 1                                                         ______________________________________                                        Homogeneous Catalysts                                                         ______________________________________                                        H.sub.4 Ru.sub.4 (CO).sub.12, Fe(CO).sub.5, Rh.sub.6 (CO).sub.16,             Co.sub.2 (CO).sub.8,                                                          (Ph.sub.3 P).sub.2 Rh(CO)H, H.sub.2 PtCl.sub.6, nickel                        cyclooctadiene, Os.sub.3 (CO).sub.12, Ir.sub.4 (CO).sub.12,                   (Ph.sub.3 P).sub.2 Ir(CO)H, NiCl.sub.2, Ni(OAc).sub.2, Cp.sub.2 TiCl.sub.2    (Ph.sub.3 P).sub.3 RhCl, H.sub.2 Os.sub.3 (CO).sub.10, Pd(Ph.sub.3            P).sub.4,                                                                     Fe.sub.3 (CO).sub.12, Ru.sub.3 (CO).sub.12, transition metal                  hydrides, transition metal salts (e.g., ZnCl.sub.2,                           RuCl.sub.3, NaHRu.sub.3 (CO).sub.11) and derivatives, PdCl.sub.2,             Pd(OAc).sub.2, (ΦCN).sub.2 PdCl.sub.2, [Et.sub.3 SiRu(CO).sub.4           ].sub.2,                                                                      (Me.sub.3 Si).sub.2 Ru(CO).sub.4, [Me.sub.2 SiXSiMe.sub.2 ]Ru(CO).sub.4,      and                                                                           mixtures thereof.                                                             ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Heterogeneous Catalysts                                                       ______________________________________                                        Pt/C, Pt/BaSO.sub.4, Cr, Pd/C, Co/C, Pt black, Co                             black, Ru black, Ra-Ni, Pd black, Ir/Al.sub.2 O.sub.3,                        Pt/SiO.sub.2, Rh/TiO.sub.2, Rh/La.sub.2 O.sub.3, Pd/Ag alloy,                 LaNi.sub.5, PtO.sub.2, and mixtures thereof.                                  ______________________________________                                    

The reaction is carried out in solution with the solvent comprisingeither the reactants themselves or an added nonreactive organic solventsuch as a hydrocarbon, an ether (e.g., ethyl ether, tetrahydrofuran), ahalogenated hydrocarbon (CHCl₃, Ch₂ Cl₂, ClCHF₂, ClCH₂ CH₂ Cl), anaromatic such as benzene, toluene, or methylphenyl ether, or a polarsolvent such as THF, acetonitrile, pyridine, or a tertiary amine. Somereactions may, if desired, be carried out in the gas phase by flowingthe reactant(s) over the transition metal catalyst.

Mild temperatures that will activate the catalyst are typically used.Such temperatures will normally be in the range of -78° C. to 250° C.,as described in U.S. Ser. No. 012, 874. In the particular reactionsdescribed and claimed herein, milder temperatures, on the order of roomtemperature or higher --i.e., about 0° C. to 150° C., more preferablyabout 20° C. to 90° C.

Reaction in ammonia will give the nitrogen-bridged structure representedby (I), while reaction in a substituted amine such as monomethylaminegives the structure represented by (II) in which the cyclomers are boundto each other directly, without a bridging atom.

The following schemes illustrate specific examples of the polymerizationreactions that give polysilazanes containing structures (I) and/or (II):##STR13##

The suggested structure obtained in the above schemes are based on acombination of ¹ H NMR, IR and elemental analyses. In contrast to thedehydrocyclodimerization (DHCD) reaction described in, inter alia, U.S.Pat. No 4,482,699 to Seyferth et al., calculations carried out byapplicants suggest that the loss of Si--H bonds is closer to one per"precursor" molecule rather than two for the DHCD reaction. Also, thepolymer produced from the reaction of Scheme VIII, although veryviscous, is still a liquid, despite the rather high molecular weight(M_(n) about 2030 D).

The reaction with ammonia (Scheme VI) generates polymers that havesoftening or melting points. These melting points can be controlled bythe reaction conditions and directly correspond to molecular weight. Toapplicants' knowledge, these are the first reported polymers with unitsof [MeSiHNH]_(n) that do have a measurable melting point. Since themolecular weights are as high as those obtained by DHCD, the polymers'melting points indicate higher structural flexibility and perhaps higherlinearity of the polymer. Because of its increased latent reactivity,the higher flexibility of the polysilazane obtained in Scheme VI doesnot cause a decrease in ceramic yields after pyrolysis (80 and 85% underN₂ and NH₃ respectively).

The polymerization of CMS in the presence of monomethylamine (SchemeVII) is slowed relative to the ammonia reactions and is similar to thepolymerization without any additional amine (Scheme VI). However, aquantitative addition of monomethylamine to the polymer as pendantgroups was observed and confirmed by ¹ H NMR and elemental analysis.These groups increase the viscosity of the product as well as the latentreactivity. This latent reactivity is observed in the faster gelationprocess when the solvent volume is reduced and the higher ceramic yieldsthat are obtained relative to the product of reaction Scheme VI.

The polymer obtained in Scheme VIII may be further reacted to give aco-polymer of the structures obtained in Schemes VI and VII, i.e., withnitrogen bridges present linking some of the cyclomers and not presentbetween others.

D. Pyrolysis to Ceramic Materials

Another important advantage of the compositions and methods of thepresent invention is the specificity and degree of ceramic yield uponpyrolysis Generally, an increase in the polymer nitrogen content resultsin higher nitrogen content and lower carbon content in the ceramicproduct.

Silicon nitride may be provided with Si and N content higher than about80 wt % upon pyrolysis of the polysilazanes provided herein whenpyrolysis is conducted under nitrogen, argon or other inert atmosphere,or higher than about 95% upon pyrolysis of the polysilazanes in anammonia or other amine atmosphere. Carbon-free polysilazanes which maybe prepared according to the method herein may provide silicon nitrideof even higher purity, i.e., 98-99% or higher.

Procedurally, pyrolysis, according to the preferred method of thepresent invention, is carried out as follows A polysilazane prepared asdescribed above is heated in a preferably inert atmosphere such as innitrogen or argon at a predetermined heating rate. If desired, pyrolysismay also be carried out in a reactive atmosphere, e.g., under NH₃, H₂O₂, H₂ O, N₂ O, an alkylamine, or the like. As demonstrated in Examples31 and 32 of U.S. Ser. No. 012,874, the heating rate during pyrolysis isstrongly correlated to the yield of ceramic material obtained. Preferredheating rates for bulk pyrolysis are between about 0.1° C. and 10° C.per minute, preferably between about 0.5° C. and 2° C. per minute, witha particularly effective heating rate, optimizing ceramic yield, ofabout 0.5° C. per minute. In some applications, however, flash pyrolysismay be preferred (e.g., in coating applications). The temperature of thepolymer is typically raised to between about 500° C. and about 900° C.,optionally higher, to about 1600° C .-1800° C., to providecrystallization, sintering or grain growth of the ceramic material. Theheating process may include one or more isothermal holding steps, inorder to control the pyrolysis, to provide more crosslinking at moderatetemperature (less than about 400° C.) and to further increase the yieldof the final product. If desired, pyrolysis may be carried out in thepresence of a catalyst; examples of suitable catalysts are set forth inTables 1 and 2. These tables are intended for illustrative purposes onlyand are not intended to limit the catalysts which could be used hereinto those recited.

Optionally, pyrolysis may be carried out only partially, i.e., inapplications where it is not necessary to obtain a fully pyrolyzedmaterial Such applications include coatings, or silazane rubbers,glasses, etc., or where the environment of a material can be damaged byhigh temperatures. Such "partial pyrolysis" or partial curing may becarried out at temperatures lower than 500° C.

Depending on the preceramic polymer pyrolyzed, then, the ceramicproducts may include silicon nitride, silicon carbide, and siliconnitride/silicon carbide alloys.

E. Ceramic Coating Procedures

The ceramic materials provided herein are useful in a number ofapplications, including as coatings for many different kinds ofsubstrates.

Silicon nitride and silicon carbide coatings may be provided on asubstrate, for example, by a variation of the pyrolysis method justdescribed A substrate selected such that it will withstand the hightemperatures of pyrolysis (e.g. metal, glass, ceramic, fibers, graphite)is coated with a preceramic polymer material by dipping in a selectedsilazane polymer solution, or by painting, spraying, or the like, withsuch polymer solution, the solution having a predeterminedconcentration, preferably between about 0.1 and 100 wt. %, morepreferably between about 5 and 10 wt. % for most applications. Thepolymer is then pyrolyzed on the substrate by heating according to thepyrolysis procedure outlined above. In such a method, pyrolysis can beconducted relatively slowly, i.e., at a heating rate between about 0 1°C. and 2.0° C. per minute, in order to allow evolved gas to escapewithout forming bubbles in the coating, and can include one or moreisothermal holding steps. In some instances, for example, withrelatively temperature-sensitive materials, or where a rapid-coatingprocess is desired, a flash pyrolysis step may be preferred. Repeated,multiple coatings may be applied where a thicker layer of material isdesired, with partial curing or gradual or flash pyrolysis followingeach individual coating step.

The pyrolysis temperature will vary with the type of coating desired.Typically, temperatures will range from about 350° C. to about 1100° C.Lower temperatures, below about 500° C., can result in only partiallypyrolyzed polymer.

Optionally, the liquid or dissolved polymer may be admixed with ceramicpowders such as silicon nitride or silicon carbide optionally admixedwith sintering aids such as aluminum oxide, silica, yttrium oxide, andthe like, prior to coating. Crosslinking agents may be included in thecoating mixture as well.

The above coating procedure is a substantial improvement over theconventional, chemical vapor deposition (CVD) method of producingsilicon nitride coatings in which the appropriate compounds (e.g., SiH₄and NH₃ or volatile silazane) react in the vapor phase to form theceramic which deposits on the target substrate. CVD is typically aninefficient, time-consuming process which requires costly andspecialized equipment The procedure described above for producingcoatings containing silicon nitride can be done with a conventionalfurnace. Further, the method leads to heat-stable, wear-, erosion-,abrasion, and corrosion-resistant silicon nitride ceramic coatings.Because silicon nitride is an extremely hard, durable material, manyapplications of the coating process are possible. One specificapplication is in gas turbine engines, on parts which are particularlysusceptible to wear. Also, because silicon nitride is an insulator, thecoating process could be used as the dielectric material of capacitors,or for providing insulating coatings in the electronics industry. Otherapplications are clearly possible.

In an alternative embodiment of the invention, a substrate isspray-coated with ceramic or preceramic materials. Such a procedureprovides for a higher density coating, as well as for a greater degreeof homogeneity. Preceramic coatings may be provided on a substrate, orat higher temperatures, one or more ceramic coatings may be provided.Gaseous species, such as silane and ammonia, which are capable ofreacting to form preceramic polymers, are introduced into a nozzle. Thegases are admixed within the nozzle and passed over a transition metalcatalyst bed contained within the nozzle, suitable transition metalcatalysts herein are selected from those set forth in Table I and II.The catalyst bed initiates the formation of preceramic materials fromthe gaseous species. At the nozzle, the gaseous preceramic materials aremixed with inert or reactive gases introduced into the apparatus throughone or more inlets, and the substrate surface is coated with a mixtureof these materials. Such a procedure, which provides a suspension ofliquid preceramic materials in air, may be used for the preparation offine powders, as well.

The desired polymerization reaction designated as type (a) or type (b)thus takes place as the gases are passed over the catalyst bed. Theinert gas delivers the preceramic materials to the substrate surface,or, if the gas phase is heated, it can deliver actual ceramic powders ormixtures of powders and preceramic polymer having controlled-sizeparticles. The process can be used to form ultrafine aerosols ofprecursors and homogeneous catalyst solutions for ultrafine particleapplications.

F. Fabrication of Molded Ceramic Bodies

The preceramic polymers are provided herein, admixed with ceramicpowders, may be used to form three-dimensional articles by injection- orcompression-molding using procedures substantially as described inco-pending application Ser. No. 012,874. The results as demonstrated inthe examples of that application indicate that the procedure may also besuccessful in the absence of sintering agents.

G. Preparation of Fibers

The polymers provided in the present invention can be used forpreceramic fiber spinning.

Three general spinning techniques are commonly used: (a) melt spinning,in which the polymer is spun from its melt and solidified by cooling;(b) dry spinning, in which the polymer is at least partially dissolvedin solution and pulled out through the spinneret into a heat chamber,then solidified by solvent evaporation; and (c) wet spinning, in which aconcentrated polymer solution is spun into a coagulation or regenerationbath containing another solvent in which the polymer is not soluble. Inaddition, gel-type polymers can be spun from very viscous solutions.These tractable polymers rapidly gel and crosslink upon removal ofsolvent after spinning due to high latent reactivity. Polymeric fibersso formed are intractable.

Additional, relatively small quantities (0.1-5.0 wt. %) of a very highmolecular weight substantially linear organic polymer(100,000-5,000,000D) may be mixed with the inorganic polymer to supportand improve the fiber strength after spinning, as taught in, e.g., U.S.Pat. Nos. 3,853,567 to Verbeek and 3,892,583 to Winter et al.

The supporting technique is especially useful when low molecular weightand/or nonlinear polymers having a very low degree of chain entanglementare used.

One problem encountered in ceramic fiber fabrication derives from thefusability of inorganic polymers during pyrolysis. This fusabilityresults in structural problems in the spun fiber. Polymers produced bythe present invention, however, overcome the fusability problem,providing that the catalytic process as described herein is actuallyincorporated into the fiber-spinning process For example, a highmolecular weight polysilazane may be mixed with homogeneous catalyst andheated in spinneret or in the curing chamber to cause reactions of type(a) or (b) or both to occur and increase the degree of crosslinking inthe fiber. Alternatively, the spinneret can itself be a catalytic bed.Crosslinking agents may also be included in the fiber-spinning processto provide additional crosslinking; similarly, latent reactive groups(e.g., free amino moieties) may be present, as well, for the samereason, even in the absence of catalyst Alternatively, the spun fibercan be exposed to an ammonia environment and cured at a temperaturebelow the melting point.

H. Other Applications

Many other applications of the novel polymers of the invention areclearly possible.

The results summarized in part F, for example, suggest combination ofpolysilazanes and related compounds with other ceramic powders (e.g.,SiC, BN, B₄ C) to produce composite articles. Such a composite of, e.g.,a silazane polymer/SiC powder mixture may give an article havingimproved oxidation resistance. Another application would be to use thenovel polymers in dissolved or liquid form as binders combined withceramic powders so as to provide a fluid polymer/powder solution.

Infiltration and impregnation processes are additional possibilities, asdiscussed, for example, in U.S. Pat. No. 4,177,230 to Mazdiyasni et aland in W. S. Coblenz et al in Emergent Process Methods forHigh-Technology Ceramics,ed. Davis et al (Plenum Publishing, 1984). Twogeneral methods are typically used. One is a high-vacuum technique inwhich a porous ceramic body is placed in a liquid or slightly dissolvedpreceramic polymer solution. After a high vacuum infiltration, thearticle is pyrolyzed to achieve a higher density. The second method ishigh-pressure infiltration. Either of these methods can be adapted forthe polymers of the invention. Either of these methods can be adaptedfor the polymers of the invention. In addition, low molecular weightoligosilazane solutions having higher mobility in the porous ceramicbody can be incubated with the ceramic body and a transition metalcatalyst, followed by incubation of the oligomeric reactants andreaction of type (a) or (b) or both. In situ chain extension orcrosslinking will reduce the mobility and volatility of the oligomericstarting materials.

Other applications of the novel polymers include use as a cement to"bond" ceramic materials such as powders, ceramic fibers, andthree-dimensional forms. Bonding of fibers followed by pyrolysis canyield matrices or matrix composites. In some application, ceramicarticles may be joined by the polymers, under pressure, followed bypyrolysis. The chemical interactions discussed in part F between thepolymer and ceramic powders may occur in bonding to enhance the strengthof the cement.

It is to be understood that while the invention has been described incon]unction with the preferred specific embodiments thereof, that theforegoing description as well as the examples which follow are intendedto illustrate and not limit the scope of the invention, which is definedby the scope of the appended claims. Other aspects, advantages andmodifications within the scope of the invention will be apparent tothose skilled in the art to which the invention pertains.

EXAMPLES

Experimental: Unless otherwise indicated, the reagents used wereobtained from the following sources silanes, from Petrarch Systems, Inc, Bristol, Pennsylvania; organic reagents including amines, from AldrichChemical Co., Milwaukee, Wisconsin; gases, from Matheson, Seacaucus, NewJersey; and catalysts, from Strem, Newburyport, Massachusetts.

EXAMPLE 1

Preparation of Cyclomethylsilazane (CMS) (-78° C.): A 2 liter 3-neckflask fitted with an overhead stirrer, nitrogen inlet, and a rubberseptum was well flushed with nitrogen. 500 ml anhydrous ether (from afreshly opened can) were added followed by 156.0 ml (172.5 g, 1.5 moles)Me(H)SiCl₂. The solution was cooled to -78° C. with a dry ice acetonebath Ammonia, dried by passing through KOH, was bubbled through thereaction for 4 hours. A large amount of white precipitate was formed.The reaction was allowed to come to room temperature with the NH₃ purgeover two more hours then filtered in a dry box. The white precipitatewas extracted with 3×350 ml boiling THF, filtered, and the combinedfiltrates were reduced to approx. 300 ml. NH₃ was bubbled through thisfiltrate for 30 minutes at room temperature to ensure complete reaction.The cloudy mixture was re-filtered through a sintered glass filter toremove the additional white solid. A very pale yellow liquid (70.3 g,79%) remained after solvent removal The GC-MS of this liquid showed themajor volatile species to be cyclomers of the structure --[CH₃Si(H)NH]--_(n) and cyclomers with side groups. There is some evidence ofrearrangement reactions on the silicon atoms during the process TheM_(n) was 720 d. The ¹ H NMR of the oligomer shows a ratio of SiH: NH:SiCH₃ (0.9: 0.6: 3.0). Elemental Analysis %C 23.29, %H 8 48, %N 23.18,%Si 38 08, %Cl < 0 1. Theoretical for [--CH₃ Si(H)NH--]: %C 20.34, %H8.47, %N 23.73, %Si 47.46. This compound gave a ceramic yield of 35 wt %after pyrolysis under nitrogen to 850° C.

EXAMPLE 2

Preparation of Cyclomethylsilazane (CMS) (0° C.): A 3-neck, 1000 mlflask fitted with an overhead stirrer and rubber septa was well flushedwith nitrogen. 500 ml anhydrous ether (from a freshly opened can) wereadded followed by 250 ml (276 g, 2.40 moles) Me(H)SiCl₂. Ammonia, driedby passing through KOH, was bubbled through the reaction at 0° C. for 4hours then at room temperature for 4 hours. A large amount of whiteprecipitate formed. The reaction was then stirred under nitrogen at roomtemperature for 2 hours. One half of the reaction was filtered in thedry box. When NH₃ was bubbled through this filtrate, more precipitateformed, indicating that the reaction was incomplete. The NH₃ was bubbledfor several more hours at room temperature until no more precipitationwas apparent The reaction was re-filtered to remove the additional whitesolid. All the white precipitate was extracted with 3×300 ml boilingTHF, filtered, and the combined filtrates were reduced under vacuum togive a very pale yellow liquid (44.5 g). The GC-MS of this liquid showedthe major volatile species to be cyclotetramers with a m/e of 236. TheM_(n) was 1134 D (some drift). ¹ H NMR (CDCl₃): 4.82, 4.62, 4.37 (m,0.92 H, SiH), 0.85 (m, 0.84 H, NH), 0.16 (bs, 3.0 H, SiCH₃). ElementalAnalysis %C 20.82, %H 8.39, %N 22.02, %Si 42.17, %Cl < 0.1. Theoreticalfor [--(CH₃)Si(H)NH--]: %C 20.34, %H 8.47, %N 23.73, %Si 47.46.

EXAMPLE 3

Preparation of Cyclomethylsilazane (CMS) (0° C.): A 3-neck, 1000 mlflask fitted with an overhead stirrer and rubber septa was well flushedwith nitrogen. 450 ml anhydrous ether (from a freshly opened can) wereadded followed by 50 ml (55.25 g, 0.480 moles) Me(H)SiCl₂. Ammonia,dried by passing through KOH, was bubbled through the reaction at 0° C.for 2 hours. A large amount of white precipitate formed. The reactionwas then stirred under nitrogen at room temperature for 12 hours thenfiltered in the dry box. All the white precipitate was extracted with5×80 ml dry ether, filtered, and the combined filtrates were reducedunder vacuum to give a colorless liquid. The M_(n) was 633 D. ¹ H NMR(CDCl₃): 4.82, 4.64, 4.39 (m, 0.74 H, SiH), 0.80 (m, 0 74 H, NH), 0.18(bs, 3.0 H, SiCH₃).

EXAMPLE 4

Preparation of Cyclomethylsilazane (CMS) (-78° C.): A 1 liter, 3-neckflask fitted with an overhead stirrer, nitrogen inlet, and a rubberseptum was well flushed with nitrogen. 300 ml anhydrous ether (from afreshly opened can) were added followed by 30.0 ml (33.15 g. 0.288moles) Me(H)SiCl₂. The solution was cooled to -78° C. with a dry iceacetone bath. Ammonia, dried by passing through KOH, was bubbled throughthe reaction for 4 hours. A large amount of white precipitate formed.The reaction was allowed to come to room temperature with the NH₃ purgeover 3 hours then stirred under nitrogen for 14 hours. A whiteprecipitate was collected by filtration then extracted with 3×150 mlboiling THF. The combined filtrates were reduced to a colorless liquid.The M_(n) was 780 D.

EXAMPLE 5

Polymerization of CMS with Ru₃ (CO)₁₂ (No solvent): 3.0 g CMS (fromExample 1) and 6.0 mg (0.00936 mmoles) Ru₃ (CO)₁₂ were added undernitrogen to a quartz reactor. The mixture was heated at 60° C. withmagnetic stirring. During the reaction, the pressure was releasedapprox. every 2.5 hours. Solvent was removed from the pale,yellow-orange solution under

vacuum after 11 hours to give a very viscous oil. The ¹ H NMR indicatedthat the polymer had a ratio of 0.71 H: 0.56 H: 3H for SiH: NH: SiCH₃.Elemental Analysis: %C 22.26, %H 7.62, %N 22.75, %Si 37.72. Thepolycyclomethylsilazane (PCMS) produced a 72 wt % ceramic yield afterpyrolysis under nitrogen and 77 wt % under NH₃. Elemental Analysis(after pyrolysis): %C 16.27, %H 0.84, %N 25.54, %Si 51.80.

EXAMPLE 6

Polymerization of CMS with Ru₃ (CO)₁₂ (THF): 5.0 g CMS (from Example 2),10.0 mg (0.0156 mmoles) Ru₃ (CO)₁₂, and 5.0 g THF were placed in astainless steel pressure reactor under nitrogen. The reaction was heatedat 60° C. with magnetic stirring. During the reaction, the pressureincreased 180 psi. The pressure was released after 10 hours and solventwas removed from the pale, yellow-orange solution under vacuum to give amoderately viscous oil with a M_(n) of 820 D. Thepolycyclomethylsilazane (PCMS) produced a 57 wt % ceramic yield afterpyrolysis under nitrogen ¹ H NMR (CDCl₃): 4.79, 4.68, 4.41 (m, 0.71 H,SiH), 0.89 (m, 0.56 H, NH), 0.19 (bs, 3 H, SiCH₃).

EXAMPLE 7

90° C. Polymerization of CMS (1/10 amount catalyst): 5.0 g CMS (fromExample 2) and 1.0 mg (0.0015 mmoles) Ru₃ (CO)₁₂ were combined undernitrogen in a quartz reactor The reaction was heated at 90° C. andmagnetically stirred After 30 hours a very viscous oil is formed. ¹ HNMR (CDCl₃): 4.65, 4.40 (m, 0.69 H, SiH), 0 86 (m, 0.56 H, NH), 0.16 (m,3 H, SiCH₃). This polycyclomethylsilazane (PCMS) produced a 63 wt %ceramic yield after pyrolysis under nitrogen. The ceramic yield afterpyrolysis under NH₃ (to 850° C.) was 68 wt %. The M_(n) was 1900 D.

The identical reaction carried out in the presence of [Et₃ SiRu(CO)₄ ]₂as catalyst gave a very viscous polymer with an M_(n) of 1650 D.

EXAMPLE 8

Polymerization of CMS with NH₃ (90°): 5.0 g CMS (from Example 1) and10.0 mg (0.0156 mmoles) Ru₃ (CO)₁₂ were dissolved in 5.0 g THF and thesolution was placed in a stainless steel pressure reactor undernitrogen. Two equivalents of ammonia (130 psi) were added and thereaction was heated at 90° C. with magnetic stirring. After 2.5 hours,the pressure was released The solvent was removed from the reactionunder vacuum producing a sticky, pale yellow wax. This material had amelting point at 10° C. and a M_(n) of 1760 D. Elemental Analysis: %C22.13, %H 7.38, %N 24.37, %Si 45 71. The material produced an 80 wt %ceramic yield after pyrolysis under nitrogen at 850° C. and 85 wt %under NH₃. Elemental Analysis (of pyrolysis product): %C 14.44, %H 0.58,%N 25 74, %Si 52 67. ¹ H NMR (CDCl₃) 4.80, 4.70, 4.42 (m, 0.62 H, SiH),0.88 (m, 0.73 H, NH), 0.21 (bs, 3.0 H, SiCH₃).

EXAMPLE 9

Polymerization of CMS with NH₃ (60°): 5.0 g CMS (from Example 2) and10.0 mg (0.0156 mmoles) Ru₃ (CO)₁₂ were dissolved in 5.0 g THF and thesolution was placed in a stainless steel pressure reactor undernitrogen. Two equivalents of ammonia (130 psi) were added and thereaction was heated at 60° C. and magnetically stirred. After 5 hours,the pressure had increased to 260 psi. The pressure was released andmore NH₃ (130 psi) was added. After 5 more hours, an additional pressureof 70 psi had accumulated. As the solvent was removed from the reactionunder vacuum a waxy solid is obtained by scraping. This solid had amelting point at 45°-55° C. and a molecular weight of 2210 D. ¹ H NMR(CDCl₃): 4.77 (m, 0.34 H, SiH), 0.89 (m, 0.97 H, NH), 0.18 (m, 3 H,SiCH.sub. 3). The solid produced a 79 wt % ceramic yield after pyrolysisunder nitrogen. Elemental Analysis (of pyrolysis product) %C 10.56, %H0.45, %N, 27.73, %Si 52.40.

EXAMPLE 10

"Curing" of APCMS (50° C.): Some of the product from Example 9 wasdissolved in an equivalent weight of THF and stirred at 50° C. for 12hours under nitrogen. The product after solvent removal was a solid witha softening point at 45°-55° C. and a melting point at 80°-90° C. Themolecular weight had increased to 2400 D.

example 11

Increased Polymerization of APCMS (15 hours): A reaction mixture ofamino-polymethylcyclosilazane (APCMS) was synthesized as in Example 9.(After 10 hours, the M_(n) was 1930 D). The product, a sticky, solublewax, was reacted with 130 psi of NH₃ at 60° C. After 5 more hours (15hours total), only 110 psi had been generated. The solvent was thenremoved under vacuum, leaving a soft rubber. The ceramic yield of therubber was 80 wt %.

EXAMPLE 12

"Curing" of APCMS at 60° C.: Some of the product from Example 11 after10 hours of reaction was dissolved in an equivalent weight of THF andstirred at 60 ° C. for 4 hours under nitrogen. The product after solventremoval was a solid with a softening point at 50°-60° C. and a meltingpoint at approx. 80° C. The molecular weight had increased to 2300 D.

EXAMPLE 13

Polymerization of CMS with MeNH₂ (60°): To 5.0 g THF in a stainlesssteel pressure reactor was added 5.0 g CMS (from Example 1) and 10.0 mg(0.0156 mmoles) Ru₃ (CO)₁₂ under nitrogen. Two equivalents of MeNH₂ (130psi) were added in portions and the reaction was heated at 60° C. withstirring for 5 hours. The pressure at this time had increased to 250psi. The atmosphere above the reaction was released. The solvent wasremoved from the reaction under vacuum to give a viscous, pale yellowoil. ¹ H NMR (CDCl₃): 4.80, 4.66, 4.42 (m, 0.61 H, SiH), 2.48 (bs, 0.63H, NCH₃), 0.80 (m, 0.78 H, NH), 0.17 (bs, 3.0 H, SiCH₃).

EXAMPLE 14

Polymerization of CMS with MeNH₂ (60° C.): To 5.0 g THF in a stainlesssteel pressure reactor was added 5.0 g CMS (from Example 2) and 10.0 mg(0.0156 mmoles) Ru₃ (CO)₁₂ under nitrogen. Two equivalents of MeNH₂ (130psi) were added in portions and the reaction was heated at 60° C. withstirring for 5 hours. The pressure at this time had increased to 180psi. The atmosphere above the reaction was released and another 130 psiof MeNH₂ was added. After 5 more hours (10 hours total), only 50 psi ofadditional pressure had developed. The solvent was removed from thereaction under vacuum to give a very viscous, pale yellow oil. Themolecular weight (M_(n)) of this oil was 830 D. ¹ H NMR (CDCl₃): 4.69(m, 0.36 H, SiH), 2.46 (bs, 0.47 H, NCH₃), 0.65 (m, 0.96 H, NH), 0.16(m, 3 H, SiCH₃). Elemental Analysis %C 23.40, %H 7.25, %N 23.17, %Si33.42. The polymer was pyrolyzed under nitrogen to give a ceramic yieldof 67%. Elemental Analysis (of pyrolysis products): %C 13.76, %H 0.51,%N 26.81, %Si s49.90.

EXAMPLE 15

Polymerization of CMS with MeNH₂ : 5.0 g CMS (from Example 3), 10.0 mg(0.0156 mmoles) Ru₃ (CO)₁₂, and 5.0 g THF were added to a stainlesssteel pressure reactor under nitrogen. 2 equivalents of MeNH₂ (140 psi)were added in portions and the reaction was heated at 60° C. withstirring. After 2.5 hours, the pressure reached a maximum at 280 psi.After 3 hours, the solvent was removed from the reaction to give a veryviscous oil which had a M_(n) of 1394 D. ¹ H NMR (CDCl₃): 4.81, 4.64 (m,0.48 H, SiH), 2.44 (bs, 0.20 H, NCH₃), 0.80 (m, 0.69 H, NH), 0.14 (m, 3H, SiCH₃).

EXAMPLE 6

"Curing" of MAPCMS at 60° C.: The oil produced in Example 15 was dilutedwith 1.5 g THF and reheated to 60° C. for 6 hours. The product was nowalmost a gel after solvent removal. This polymer was pyrolyzed undernitrogen to give a ceramic yield of 67 wt %. ¹ H NMR (CDCl₃): 4.70 (m,0.42 H, SiH), 2.46 (bs, 0.18 H, NCH₃), 0.83 (m, 0.59 H, NH), 0.14 (m, 3H, SiCH₃).

EXAMPLE 17

Polymerization of CMS with PdCl₂ (C₆ H₅ CN)₂ under NH₃ (60°): 5.0 g CMS(from Example 2), 6.0 mg (0.0156 mmoles) PdCl₂ (C₆ H₅ CN)₂, and 5.0 gTHF were added to a stainless steel pressure reactor under nitrogen. 130psi of ammonia were added and the reaction was heated at 60° C.magnetically. After 5 hours, the pressure had increased by 60 psi. Thepressure was released and 130 psi of NH₃ was added. After 5 more hours,only 30 psi pressure had accumulated. A slightly viscous black oil wasobtained after removing the solvent from the reaction. This oil had amolecular weight of 976 D. ¹ H NMR (CDCl₃): 4.83, 4.65, 4.40 (m,

0.74 H, SiH), 0.83 (m, 0.88 H, NH), 0.16 (m, 3 H, SiCH₃). ElementalAnalysis: %C 21.86, %H 7.68, %N 5.01, %Si 42.39.

EXAMPLE 18

Polymerization of CMS in the presence of (hexyl)SiH₃ at 60° C.: 10.0 mg(0.0156 mmoles) Ru₃ (CO)₁₂ were added to 5.0 g CMS (from Example 4) and2.0 g (hexyl)SiH₃ (17.2 mmoles). The reaction was stirred for 50 hoursat 60°-90° C. Solvent removal after this time yielded only a slightlyviscous green liquid. ¹ H NMR (CDCl₃): 4.67, 4.36 (m, 0.94 H, SiH), 3.46(m, 0.12 H, SiHx), 1.28 (m, 0.97 H, CH₂), 0.86 (m, 0.36 H, CH₃), 0.22(m, 0.24 H, SiCH₂), 0.18 (m, 3.0 H, SiCH₃). The M_(n) of the polymer was1450 D.

EXAMPLE 19

Precursor Formation: Into a flame-dried three-neck flask equipped withan overhead mechanical stirrer and an N₂ inlet was placed 500 mlanhydrous ether. This was cooled to <-70° C. in a dry ice/acetone bath.Dichlorosilane (150 g; 1.5 moles) was then condensed into the flask. Anexcess of about 198 g (4.5 moles) monoethylamine was then added over atwo-hour period. The reaction mixture was stirred for an additional fourhours, and the flask was then allowed to warm slowly overnight to roomtemperature. The contents were diluted with 500 ml ether and filtered toremove monoethylamine hydrochloride salt.

The solids were then placed in a 2 1 Erlenmeyer flask and stirred for 10minutes in 500 ml boiling THF. The mixture was filtered hot. Theextraction was repeated and the solids were rinsed with an additional500 ml hot THF. 89.0 g of products were obtained after solvent removal(81% yield with M_(n) =490 D). Fractionation by high vacuum distillation(150° /300 μ) gives 60% of volatile products having M_(n) of 307 D and40% residue with M_(n) =420 D.

EXAMPLE 20

Precursor Polymerization with NH₁ (90° C.): 5.0 g of the precursor[EtN-SiH₂ ]_(n) prepared in Example 19 and 10.0 g (0.0156 mmoles) Ru₃(CO)₁₂ were placed in a stainless steel pressure reactor under nitrogen.Two equivalents of ammonia (160 psi) were added and the reaction mixturewas heated at 90° C. with magnetic stirring. After 6 hours, the pressurewas released The product, a very viscous liquid, was divided prior topyrolysis in order to pyrolyze both under nitrogen and under ammonia.The ceramic yields obtained were 84 wt % (N₂) and 86 wt % (NH3).Elemental analysis (in mole ratios) was as follows:

    ______________________________________                                                   C     H       N       Si    O                                      ______________________________________                                        Polymer before                                                                             1.42    5.69    1.33  1.00  0.11                                 pyrolysis                                                                     Polymer after                                                                              0.48    0.58    1.05  1.00  0.05                                 pyrolysis under NH.sub.3                                                      Polymer after                                                                              0.24    0.55    1.44  1.00  0.03                                 pyrolysis under NH.sub.3                                                      ______________________________________                                    

EXAMPLE 21

Polysilazane Coatings: Coatings of the polysilazane precursor of Example5 were prepared by dipping soda lime rods, 3 mm in diameter, intopolysilazane solutions (M_(n) of about 2000 D) in THF havingconcentrations of 2.5 wt.%, 5 wt.% and 10 wt.%. The samples were curedunder a slow pyrolysis regime (heating rate of about 100° C./hr) to afinal temperature of 590° C. The cured coatings were transparent andsmooth. The thickness of the cured coating was measured by SEM and foundto be in the range of 0.1 and 0.5 microns for the 2 5 wt.% and 5 wt.%solutions and in the range of 1.0 and 4 0 microns for the 10 wt.%solution

EXAMPLE 22

Fiber Preparation: Fibers of about 4" to 24" were formed from thepolymer of Example 8, a soft wax, at room temperature These fibers werecured under ammonia for approximately 3 hours, again at room temperatureAfter curing, the fibers were heated under ammonia, first to 250° C. ata heating rate of 60° /hour, and then to 850° C. at a heating rate of300° /hour. The ceramic fibers obtained after pyrolysis had diameters onthe order of 4 to 50 microns, and maintained their shape without anyflaws or breakage FIGS. 1 and 2 are SEM photographs of a fiber preparedaccording to this procedure.

We claim:
 1. A method of making a ceramic composition, comprisingpyrolyzing at a temperature in the range of about 500° C. to about 1800°C. a polysilazane consisting essentially of n recurring units of thestructures ()I) or (II) or both ##STR14## wherein m is an integer, the Rsubstituents may be the same or different and are selected from thegroup consisting of hydrogen, lower alkyl, lower alkenyl, silyl, aryl,and amino, unsubstituted or substituted with 1 or 2 lower alkyl groups,X is --NR', --NR'--NR'--or --NR'--Y--NR' where Y is lower alkyl orsilyl, and the R' and R" may be the same or different and areindependently selected from the group consisting of hydrogen, loweralkyl, lower alkenyl, silyl and aryl.
 2. A method of making a ceramiccomposition, comprising pyrolyzing at a temperature in the range ofabout 500° C. to 1100° C. a polysilazane consisting essentially of nrecurring units of the structures (I) or (II) or both ##STR15## whereinm is an integer, the R substituents may be the same or different and areselected from the group consisting of hydrogen, lower alkyl, loweralkenyl, silyl, aryl, and amino, unsubstituted or substituted with 1 or2 lower alkyl groups, X is --NR', --NR'--NR----or --NR'--Y--NR' where Yis lower alkyl or silyl, and the R' and R" may be the same or differentand are independently selected from the group consisting of hydrogen,lower alkyl, lower alkenyl, silyl and aryl, wherein the polysilazane isprovided as a coating on a substrate prior to pyrolysis.
 3. A method ofmaking ceramic articles, comprising:providing a solution of a polymerconsisting essentially of n units of the structures (I) or (II) or both##STR16## wherein m is 0 or an integer, n is an integer greater than 1,the R substituents may be the same or different and are selected fromthe group consisting of hydrogen, lower alkyl, lower alkenyl, silyl,aryl, and amino, unsubstituted or substituted with 1 or 2 lower alkylgroups, X is --NR'--, NR'--NR' or --NR'--Y--NR'--where Y is lower alkylor silyl, and the R' and R" may be the same or different and areindependently selected from the group consisting of hydrogen, loweralkyl, lower alkenyl, silyl and aryl; admixing said polymer solutionwith ceramic powders, ceramic whiskers, ceramic fibers, or with a porousor nonporous ceramic article; and thermally treating said admixture at atemperature in the range of about 500° C. to 1600° C. so as to form aceramic article.
 4. The method of claim 1, wherein the polysilazanecontains n recurring units of the structure (I), and n is an integergreater than
 1. 5. The method of claim 1, wherein the polysilazanecontains n recurring units of the structure (II), and n is an integergreater than
 1. 6. The method of claim 1, wherein the pyrolyzing iscarried out in an inert atmosphere.
 7. The method of claim 6, whereinthe inert atmosphere is nitrogen.
 8. The method of claim 6, wherein theinert atmosphere is argon.
 9. The method of claim 1, wherein thepyrolyzing is carried out in a reactive atmosphere.
 10. The method ofclaim 9, wherein the reactive atmosphere is ammonia.
 11. The method ofclaim 9, wherein the reactive atmosphere is hydrogen peroxide vapor. 12.The method of claim 9, wherein the reactive atmosphere is water vapor.13. The method of claim 9, wherein the reactive atmosphere is N₂ O. 14.The method of claim 1, wherein the pyrolyzing is carried out at aheating rate of 0.1° C. to 10° C. per minute.
 15. The method of claim 1,wherein the pyrolyzing is carried out using flash pyrolysis.
 16. Themethod of claim 1, wherein the pyrolyzing is carried out in the presenceof a catalyst.
 17. The method of claim 1, wherein the pyrolyzingincludes at least one isothermal holding step.
 18. The method of claim2, wherein the substrate is of a material selected from the groupconsisting of metal, glass, ceramics and graphite.
 19. The method ofclaim 2, wherein the polysilazane is provided in a solvent, and whereinthe polysilazane is present in said solvent at a concentration ofbetween about 0.1 and about 100 wt. %.
 20. The method of claim 2,wherein the polysilazane is provided in a solvent, and wherein thepolysilazane of between about 5 and 10 wt. %.
 21. The method of claim 2,wherein said coating involves dipping the substrate in the polysilazane.22. The method of claim 2, wherein said coating involves spraying thesubstrate with the polysilazane.
 23. The method of claim 2, wherein saidcoating involves painting the substrate with the polysilazane.
 24. Themethod of claim 1, wherein the polysilazane the structure. ##STR17## 25.The method of claim 1, wherein the polysilazane contains the structure##STR18##
 26. The method of claim 1, wherein the polysilazane containsthe structure. ##STR19##
 27. The method of claim 1, wherein thepolysilazane contains the structure ##STR20##
 28. The method of claim 2,wherein the polysilazane contains the structure ##STR21##
 29. The methodof claim 2, wherein the polysilazane contains the structure ##STR22##30. The method of claim 2, wherein the polysilazane contains thestructure ##STR23##
 31. The method of claim 2, wherein the polysilazanecontains the structure ##STR24##
 32. The method of claim 19, whereinsaid pyrolyzing is carried out at a temperature in the range of about500° C. to about 900° C.
 33. The method of claim 22, wherein saidthermally treating is carried out at a temperature in the range of about500° C. to about 900° C.